net/dns: add GetBaseConfig to OSConfigurator interface.

Part of #953, required to make split DNS work on more basic
platforms.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/direct.go

@@ -163,6 +163,11 @@ func (m directManager) SupportsSplitDNS() bool {
 	return false
 }
 
+func (m directManager) GetBaseConfig() (OSConfig, error) {
+	// TODO
+	return OSConfig{}, nil
+}
+
 func (m directManager) Close() error {
 	if _, err := os.Stat(backupConf); err != nil {
 		// If the backup file does not exist, then Up never ran successfully.

net/dns/manager_test.go

@@ -36,6 +36,11 @@ func (c *fakeOSConfigurator) SupportsSplitDNS() bool {
 	return c.SplitDNS
 }
 
+func (c *fakeOSConfigurator) GetBaseConfig() (OSConfig, error) {
+	// TODO
+	return OSConfig{}, nil
+}
+
 func (c *fakeOSConfigurator) Close() error { return nil }
 
 func TestManager(t *testing.T) {

net/dns/manager_windows.go

@@ -300,6 +300,23 @@ func (m windowsManager) Close() error {
 	return m.SetDNS(OSConfig{})
 }
 
+func (m windowsManager) GetBaseConfig() (OSConfig, error) {
+	if m.nrptWorks {
+		return OSConfig{}, errors.New("GetBaseConfig not supported")
+	}
+	resolvers, err := m.getBasePrimaryResolver()
+	if err != nil {
+		return OSConfig{}, err
+	}
+	return OSConfig{
+		Nameservers: resolvers,
+		// Don't return any search domains here, because even Windows
+		// 7 correctly handles blending search domains from multiple
+		// sources, and any search domains we add here will get tacked
+		// onto the Tailscale config unnecessarily.
+	}, nil
+}
+
 // getBasePrimaryResolver returns a guess of the non-Tailscale primary
 // resolver on the system.
 // It's used on Windows 7 to emulate split DNS by trying to figure out

net/dns/nm.go

@@ -202,6 +202,11 @@ func (m nmManager) SetDNS(config OSConfig) error {
 
 func (m nmManager) SupportsSplitDNS() bool { return false }
 
+func (m nmManager) GetBaseConfig() (OSConfig, error) {
+	// TODO
+	return OSConfig{}, nil
+}
+
 func (m nmManager) Close() error {
 	return m.SetDNS(OSConfig{})
 }

net/dns/noop.go

@@ -6,9 +6,10 @@ package dns
 
 type noopManager struct{}
 
-func (m noopManager) SetDNS(OSConfig) error  { return nil }
-func (m noopManager) SupportsSplitDNS() bool { return false }
-func (m noopManager) Close() error           { return nil }
+func (m noopManager) SetDNS(OSConfig) error            { return nil }
+func (m noopManager) SupportsSplitDNS() bool           { return false }
+func (m noopManager) Close() error                     { return nil }
+func (m noopManager) GetBaseConfig() (OSConfig, error) { return OSConfig{}, nil }
 
 func NewNoopManager() noopManager {
 	return noopManager{}

net/dns/osconfig.go

@@ -17,6 +17,13 @@ type OSConfigurator interface {
 	// installing a resolver only for specific DNS suffixes. If false,
 	// the configurator can only set a global resolver.
 	SupportsSplitDNS() bool
+	// GetBaseConfig returns the OS's "base" configuration, i.e. the
+	// resolver settings the OS would use without Tailscale
+	// contributing any configuration.
+	// GetBaseConfig must return the tailscale-free base config even
+	// after SetDNS has been called to set a Tailscale configuration.
+	// Only works when SupportsSplitDNS=false.
+	GetBaseConfig() (OSConfig, error)
 	// Close removes Tailscale-related DNS configuration from the OS.
 	Close() error
 }

net/dns/resolvconf.go

@@ -142,6 +142,11 @@ func (m resolvconfManager) SupportsSplitDNS() bool {
 	return false
 }
 
+func (m resolvconfManager) GetBaseConfig() (OSConfig, error) {
+	// TODO
+	return OSConfig{}, nil
+}
+
 func (m resolvconfManager) Close() error {
 	var cmd *exec.Cmd
 	switch m.impl {

net/dns/resolved.go

@@ -157,6 +157,11 @@ func (m resolvedManager) SupportsSplitDNS() bool {
 	return false
 }
 
+func (m resolvedManager) GetBaseConfig() (OSConfig, error) {
+	// TODO
+	return OSConfig{}, nil
+}
+
 func (m resolvedManager) Close() error {
 	ctx, cancel := context.WithTimeout(context.Background(), reconfigTimeout)
 	defer cancel()

wgengine/router/callback.go

@@ -49,6 +49,11 @@ func (r *CallbackRouter) SupportsSplitDNS() bool {
 	return r.SplitDNS
 }
 
+func (r *CallbackRouter) GetBaseConfig() (dns.OSConfig, error) {
+	// TODO
+	return dns.OSConfig{}, nil
+}
+
 func (r *CallbackRouter) Close() error {
 	return r.SetBoth(nil, nil) // TODO: check if makes sense
 }